After you've seen the "new" posts (that is, the posts that Instagram thinks are new for you), it no longer shows you "old" posts.
After all, who cares if it's your timeline that you manage to see posts that you want to see. No, daddy knows best. So here are "suggested posts" with a tiny link to show the "old" posts in your actual timeline.
After after all, their in-app surveys on "how to make the app better" are invariably "how to show you more ads".